1. Field of the Invention
This invention relates to the field of digital modulators/demodulators (modems) using an electric network as a transmitting medium. The invention more specifically relates to allocating a channel to a transmission between at least two modems connected to the same electric network.
2. Discussion of the Related Art
Modems using an electric network as a transmitting medium are used in particular in home automation applications. In such applications, different electric devices (heating, lighting, alarm, etc.) connected to the network are associated with modems and thus are likely to be controlled by a central control station according to a timed scenario and/or various types of detectors (temperature, fire, inundation, house-breaking, etc.). The central station is generally associated with a system for receiving instructions transmitted by the telephone network so as to control or program scenarios remotely from the home automation installation.
In such an installation, the electric network at 50 or 60 Hz is the transmitting medium. A frequency shift keying (FSK) modulation is generally used. The data to be transmitted is encoded in a binary form, then converted into an analog signal, for example, as samples of portions of one or the other of two frequencies fa and fb which constitute the signal to be transmitted on the network. The amplitude of the modulation is low (around one volt) with respect to the voltage of the power line so as not to harm the supply of the different devices. Installations using an amplitude shift keying modulation can also be found. Here, the binary data is converted into an analog signal, as samples of portions of a frequency which constitutes the signal to be transmitted on the network.
A problem encountered with this type of transmission has to do with the use of the electric network as a transmitting medium. Indeed, the signal received by a given modem is likely to be strongly attenuated due to the transmitting medium used. This attenuation, which is variable and uncontrollable, essentially has two causes.
First, the network impedance at the frequencies used for the modulation varies according to the loads it supplies. This variation of impedance which is substantially inductive and, for example, varies between 1.5 and 80 xcexa9 at the frequency of 100 kHz, is uncontrollable since it depends on the loads supplied to the installation involved and on the impedance of the electricity dispensing transformer to which the installation is connected. Moreover, the network impedance depends on the loads supplied in all the installations that the transformer supplies. The attenuation due to these variations of impedance is, for example, between approximately 2 and 40 dB for a typically resistive load.
Second, the electric connections attenuate the signal. This attenuation is even more significant if the signal transmitted by a given modem is addressed to a modem that is connected to another branch of the electric circuit. In other words, the crossing of the prepatch panel and of the fuses and circuit-breakers it includes causes a significant attenuation of the signal (of approximately 20 to 30 dB).
It is an object of the present invention to overcame these disadvantages by providing a circuit that is able to use several transmission channels and to automatically select the best transmission channel.
It is also an object of the invention to provide an allocation circuit which requires no alteration to the structure of existing modems.
It is a further object of the invention to have the allocation circuits, which have a different number of channels, be compatible with one another without any intervention.
It is a further object of the invention to make the transmission channels user-programmable.
To achieve these objects, an embodiment of the present invention provides a circuit for allocating a channel to a transmission between at least two modems using an electric network as a transmitting medium of a binary data flow, including, on the receive side, a device for selecting a channel chosen for its high quality transmission level according to the energy received on several transmission channels, the transmission taking place, at least at the beginning of each communication, on several channels.
According to another embodiment of the present invention, the circuit includes, on the transmitting side, a device for modulating a signal to be transmitted on at least one frequency selected from among several high frequencies defining the transmission channels.
According to an embodiment of the present invention, said selection device issues a configuration signal to a demodulation device of a signal received on said selected channel.
According to another embodiment of the present invention, the selection device issues all said modulation frequencies to the modulation device while it issues a single one of these frequencies, associated with the channel selected for the communication, to the demodulation device, this single frequency constituting the configuration signal of the demodulation device.
According to an embodiment of the present invention, said modulation device includes at least two multipliers of the data flow by each of said high frequencies issued by said selection device and a mixer of the output signals of said multipliers.
According to another embodiment of the present invention, said demodulation device includes a multiplier receiving, as inputs, a signal coming out from the network after being filtered and the high frequency associated with the channel selected and issued by said selection device.
According to an embodiment of the present invention, said modulation device is associated with a switch receiving, as inputs, the different modulation frequencies, said switch being controlled according to the channel selected by said selection device.
According to an embodiment of the present invention, said selection device further issues a signal indicating the channel selected to a processor for controlling the modem, said processor sending to the allocation circuit a control signal that enables forcing of the modulation and demodulation frequencies.
According to an embodiment of the present invention, the selection of the transmission channel is performed, on the receive side, during a frame for establishing a communication sent, on all channels, by a transmitter modem.
According to an embodiment of the present invention, said signal to be transmitted is a binary data flow converted into a modulated signal in the form of samples of portions of two frequencies centered on an initial central frequency by a frequency shift keying modulator, the modulation being performed by said modulation device consisting of shifting said initial central frequency towards at least one higher frequency while respecting the distance between the two frequencies of the FSK modulation.
According to an embodiment of the present invention, the output of said multiplier of the demodulation device is sent to a band-pass filter having a bandwidth that corresponds to said distance between the frequencies of the FSK modulation and which is centered on said initial frequency, the output of said band-pass filter being sent to an FSK demodulator for restoring a binary data flow.
According to another embodiment of the present invention, said signal to be transmitted is a binary data flow converted into a modulated signal in the form of samples of portions of a frequency by an amplitude shift keying modulator, the modulation being performed by said modulation device consisting of shifting said frequency of said ASK modulator towards at least one higher frequency.